Support Consul Connect Native integration

[Walliee]

Is your feature request related to a problem? Please describe.
While it's possible to configure Spring application with a sidecar proxy to use Consul Connect today, it would nice to have native support using spring-cloud-consul.

Describe the solution you'd like
Consul Connect has three components:

• Service Discovery (support for this already exists in spring-cloud-consul)
• mTLS
• Authorization

In order to support Consul Connect Native App integration, changes need to be client and server-side:

• Consul connect enabled clients need to be configured such on outbound connections they provide service certificate as the client certificate and verify the remote certificate against the public CA roots.
• Consul connect enabled servers need to be configured such that on inbound connections they verify the client certificate against the public CA roots and verify if the connection is authorized.

Additional context
https://www.consul.io/docs/connect/native
https://www.consul.io/api-docs/agent/connect#connect-agent-http-api

I've already created an issue on the consul-api project to add Connect API support. Ecwid/consul-api#207

[spencergibb]

I've wanted to write this very issue before!

[hadrabap]

I did the implementation of Agent-local operations. Could you please take a look?

Ecwid/consul-api#225

[spencergibb]

@hadrabap I have no control over the consul-api project.

[hadrabap]

@spencergibb Sure, me neither. ;-) I just wanted feedback if the feature satisfies your needs (API) or if additional work should be done.